1. Field of the Invention
The present invention relates to a data recording apparatus, a data reproducing apparatus, etc. in which moving picture signals are compression-coded for recording or reproduction.
The present invention further relates to a data transmission apparatus in which moving picture signals are compression-coded for transmission to remote sites.
2. Description of the Related Art
Generally speaking, in systems for recording and reproducing digital moving picture signals and systems for transmitting them to remote sites, an image signal compression-coding method is adopted in order to efficiently utilize the capacity of the recording medium, the transmission line, etc. In such a method, it is general practice to perform the compression coding of moving picture signals by utilizing in-frame correlation or inter-frame correlation. Examples of the compression coding formats include MPEG1 and MPEG2. Further, coding methods in conformity with the H. 261 recommendation, etc. are available.
In compression coding apparatuses, etc. operating in accordance with such coding formats, coding is to be effected such that the amount of moving picture signal data generated during a fixed period of time is kept constant.
However, the amount of data generated by compression coding is usually not constant. Further, the amount of data thus generated differs from frame to frame.
If the amount of data generated per unit time is not constant, it is very difficult to directly record the generated data on a recording medium or directly transmit it.
This is generally due to the fact that the data recording rate or the data transmission rate is constant.
In view of this, a buffer memory is usually provided between the moving picture image compression coding section and the medium recording section or between the moving picture image compression coding section and the transmission section so that the data rate of the variable amount of coded data output to the recording apparatus, the transmission line, etc. may be kept constant. In the moving picture image compression coding section, the amount of coded data generated is adjusted such that the buffer memory will not suffer failure.
Thus, the average rate of data amount generated per unit time in the moving picture image compression coding section when performing moving picture image compression coding is the same as the data rate of the coded data output from the buffer memory.
The device for performing these operations is generally called an encoder, which includes the buffer memory.
In the device for decoding the compression-coded data to the original moving picture signals, a buffer memory similar to the one mentioned above is provided between the recording apparatus or the output end of the transmission line and the expansion decoding section. The device for performing these operations is generally called a decoder, which includes the buffer memory.
In this way, in the conventional encoder and decoder, compression coding processing is effected so that the amount of moving picture signal data per unit time may approach a constant value or it may approach a target generation amount of coded data.
Here, a case will be considered in which coded data obtained by compression-coding a moving picture signal of a time length T is recorded on a recording medium at a writing rate R, the recorded data then being read out at reading rate R and decoded to thereby reproduce the moving picture signal. In this case, the target generation amount of coded data Ei is given by the following equation: EQU Ei=R.times.T (1)
However, as shown by the following inequality, the actual amount of data generated Er is not the constant value R.times.T but varies as follows: EQU R.times.T-B.ltoreq.Er.ltoreq.R.times.T+B (2)
where B represents the capacity of the buffer memory provided in the encoder. That is, it is possible for the amount of coded data Er to vary by the capacity B of the buffer memory in the encoder.
Assuming that this data is recorded on the recording medium as it is and then read out, the reading time t is as shown by the following inequality: ##EQU1## The reproduction time for the moving picture signal which is obtained by decoding the data read out is also T. This shows that image reproduction time and the requisite time for reading out the data is not the same.
When, as in the case of an AV server, an arbitrary number of items of data obtained by compression-coding moving picture signals of a fixed period of time (they will be hereinafter referred to as "clips") are written to a medium, and an arbitrary number of clips are read out from the medium to continuously reproduce the image signals, the following problem is involved:
When an arbitrary number of clips obtained by compression-coding moving picture signals of a fixed period of time are written to a medium and an arbitrary number of clip image signals are continuously reproduced therefrom, the requisite time for reading out data differs for each item of data, so that, when the time for reading out data is longer than the time for reproduction, it is impossible to continuously reproduce the actual clip image signals without interruption.